1. Field of the Invention
The invention relates to an apparatus and method for heat stress testing canisters.
2. Description of Related Art
The U.S. Food and Drug Administration has requested that medical inhalation aerosol canister products be subjected to heat stress testing. Further, the Department of Transportation has required for any product or container having more than four ounces of propellant to undergo similar heat stress testing.
Inhalation aerosol canisters may be used to contain and administer a variety of pharmaceutical drugs. For example, these types of devices have been used to contain a bronchodilator and one or more propellants, such as Proventil® (Schering Corporation, Kenilworth, N.J.), which contains albuterol (active ingredient) and a combination of trichloromonofluoromethane and dichlorodifluoromethane (propellants).
The effects of thermal and mechanical stressing on canisters have been studied to ensure that the product could withstand recommended maximum storage temperatures (e.g., for Proventil® canisters, 49° C./120° F.-112 psig). Heat stress testing can be performed as a check on the (crimp) integrity of the canister valve. Heat stress testing can be accomplished via a variety of ways, for instance, by using a (a) water bath heater, (b) conductive heat block conveyor, (c) mechanical vacuum, (d) induction heater, (e) microwave heater, (f) infrared heat gun, (g) laser, and the like.
An induction heater (heat tunnel) utilizes radio frequencies to generate heat within conductive materials (e.g., metals, polymers, etc), such as a metal canister, where resistive heating within the metal of the canister heats the internal contents of the canister and causes thermal stressing therein. The efficacy of a canister can be tested by applying thermal stress to it. Applying heat stress to a canister increases the pressure within the canister, which can accelerate the loss of propellant(s) from grossly leaking canisters with seriously faulty crimps. It has been found, however, that the value of conventional testing is limited because the testing itself could cause damage to the valves of the canisters.
Referring to the figures, a diagram of a prior art, induction heat tunnel stress testing apparatus is shown in FIG. 1. An induction heat tunnel 100 includes an induction coil system 102 powered by a power supply 104. For example, the induction coil system 102 can be a fourteen inch copper induction heating coil, and the power supply can be a 7.5 kW DC power supply. The induction heat tunnel 100 can be used to conduct heat stress testing on a representative number of metal canisters 106 from each manufacturing batch (e.g., 10% of the batch) by raising internal pressures of the canisters 106 to desired levels, for example, about 80-90 psi (34-39° C.).
As shown in FIG. 1, the canisters 106 are conveyed through the induction heat tunnel 100 so that a longitudinal axis of the canister is in a substantially horizontal position relative to a longitudinal axis of a heating zone of the induction coil system 102. The required heat stress is achieved by adjusting the conveyor speed and the power input to the induction coil system 102. The canisters 106 are completely enclosed by and uniformly heated within the induction coil system 102, which emits electromagnetic waves to heat stress the canisters 106. The application of heat radiation can increase the internal pressure in each canister 106. This increase in pressure, caused by the thermal stress, can accelerate the loss of propellants from grossly leaking canisters with faulty crimps.
Referring again to the figures, FIG. 2 is a prior art, partial cross-sectional view of a typical, normally functional metered dose valve 10, which when actuated provides an exact defined amount of drug product to the patient, with FIG. 3 being a top plan view thereof. FIG. 4 shows an enlarged cross-sectional view of the circled portion of the valve 10 of FIG. 2. As shown therein, the valve 10 includes an annular valve body 12 with an annular projection 14. The valve body 12 is that portion of the valve 10 that provides the metered dose of drug to the patient. An annular rubber valve seat 16 sits on the annular projection 14 and provides a seal with a valve stem 18 positioned axially within the valve body 12 and the valve seat 16. The valve seat 16 is a rubber seat located at the base of the valve stem 18 and at the center of the valve body 12, and provides a seal between the valve body 12 and the valve stem 18. As shown in FIG. 3, during normal functioning of the valve 10, the inner surface of the valve seat 16 provides a seal against the outer surface of the valve stem 18.
However, as a result of thermal stress testing, the rubber valve seat 16 can become deformed around the valve stem 18, which deformation is shown in exaggerated form in FIGS. 5 (top plan view) and 6 (enlarged cross-sectional view). In the state shown in FIG. 6, the valve seat 16 forms a so-called “dog ear” deformation 17, which is a severe deformation of the rubber valve seat 16 in the shape of a dog's ear.
Furthermore, as a result of the increased heat applied to the canister (and the increased pressure generated within the canister) during thermal stress testing, the rubber valve seat 16 of the canister occasionally deforms, whereby a condition known as “blow-by” occurs. “Blow-by”, as used herein with respect to a canister, is defined as a defect classified by an irregular or continuous spray (or discharge) upon actuation of the valve 10 and/or the presence of a residue from a product that has leaked out between the valve stem 18 and the valve seat 16 (or a gasket) onto metal valve ferrule components 22 of the canister. The valve ferrule 22 is an aluminum portion of the valve 10 that holds the rubber valve seat 16 in place at the base of the valve stem 18. “Blow-by” can result from the deformation of the rubber valve seat 16 around the valve stem 18, which produces a gap at a corner of the metal valve ferrule 22. “Blow-by” can sometimes be heard as a “hollow” sound produced on actuation of the valve 10 and can also be accompanied by an irregular continuous spray (or discharge). Canisters that have been thermally stressed so as to produce “blow-by” were x-rayed imaged and often shown to have a large gap 20 at the corner of the valve ferrule 22, which was produced by the deformation of the valve seat 16.
Accordingly, this method of heat stress testing can result in the damage and/or destruction of the canister valve or other components of the canister. Thus, there is a need for an adequate in-process heat stress testing apparatus and method that does not damage the canister contents or valve components in order to ensure canister integrity and, thus, product quality for patients. It would be desirable to provide an apparatus and method for heat stress testing canisters that do not damage and/or destroy valve and/or other components of the canisters.